Oct. 03, 2011

Using Cabbage Juice as an Indicator

The chemistry of acids and bases is occurring all around us! Many household cleaning items are basic in nature, while our bodies utilize strong acids in our stomach to help digest the food we eat.

In this activity, girls learn about acids and bases by using an indicator to test items you could find at home. This summer at Girlstart, campers used cabbage juice to test various liquids such as orange juice, vinegar, laundry detergent, and more. Girls discovered that the science of pH is important in our daily lives. You can explore pH chemistry too, find items from your own pantry to do this experiment yourself!

Objectives:
• Use observation and data recording techniques.
• Understand the concept of an indicator and how to use it.
• Identify if solutions are acids or bases.

Key concepts and Vocabulary
• pH – the measure of how acidic or basic a liquid is on a scale of 1-14, or the measure of the amount of hydrogen ions in the liquid.
• Acid – solutions with a pH of 1-7, these solutions have free hydrogen ions (H+) when mixed with water
• Base – solutions with a pH of 7-14, these solutions have free hydroxide ions (OH-) when mixed with water
• Indicator – a chemical that changes color if it comes in contact with an acid or a base

Engage:
Raise your hand if you ever drank orange juice? Raise your hand if you ever drank milk? How are they different? (taste, color, etc.) Why do they taste different? (they are different substances)

We are going to test some different liquids to see if they are an acid or a base?

Ask: Does anyone know what an acid is? Does anyone know what a base is? Gather different ideas.

How should we test these liquids? We are going to use an indicator – this is a chemical that changes color when it is mixed with an acid or a base. The chemical we will use is cabbage juice.

Explore:
1. In pairs or small groups, have students/campers place a small amount of each liquid in a clear cup
2. Using a clean pipette, add 1 pipette of cabbage juice to each liquid.
3. Mix by slowly swirling the liquids, or stirring with a coffee stirrer
4. Observe the color change.

The purple cabbage juice that acts as the indicator in this experiment, it turns red when mixed with something acidic and blue or green if mixed with something basic.

Can you determine which of your liquids are acidic and basic?

Can you put them in order from most acidic to neutral to most basic based on the colors?

Evaluate:
Why is pH and chemistry important? Who uses this information?
• Careers and pH
o Environmental scientists use their knowledge of acids and bases to determine the livability of an environment because large amounts of acid are incredibly harmful to the environment.

o Marine zoologists will use pH testing to determine if the water in aquariums are healthy for the marine animals.

o Geologists test the pH of soil to help determine the types of mineral deposits present in different rock layers so they can recreate the geologic history of an area.